The present invention relates to timing circuits, and more particularly to a digital time base circuit for a time domain reflectometer that uses the difference in period between two phase-locked oscillators to achieve small time delay increments.
The purpose of a time base circuit is to provide timing signals needed to control various aspects of an instrument, such as excitation and data acquisition systems for a time domain reflectometer. The timing signals for the time domain reflectometer provide a clock signal to drive a mask generator that is phase-locked to an excitation pulse, and a clock signal that is phase-locked to a sample trigger used to control the acquisition and processing of multiple samples for each excitation pulse. It is necessary to provide small time delay increments to delay the excitation pulse relative to the samples. This has been achieved by using either analog or a combination of digital and analog techniques.
One such example of a prior art time base circuit is disclosed in the OF235 Fiber Optic Time Domain Reflectometer manufactured by Tektronix, Inc. of Beaverton, Oreg. A master clock is divided to provide the necessary clock signals for the time base circuit. An analog ramp signal is generated from an excitation pulse trigger and compared with a reference level derived from a digital value provided by a processor to generate a sample trigger. U.S. Pat. No. 4,841,497 issued on June 20, 1989 to Glenn Bateman and Donald L. Brand entitled "Digital Time Base with Corrected Analog Interpolation" also describes such a time base circuit, specifically indicating how the generated ramp signal may be calibrated to provide more precise delay times.
However these traditional methods cannot be used where there is a requirement for providing clock signals for use in other parts of the measurement system. Therefore what is desired is a digital time base system for providing small delay increments while providing clock signals to other parts of a measurement system.